The present invention relates to a polypropylene resin composition and oriented film thereof. In more particular, it relates to a polypropylene resin composition for oriented film excellent in transparency, slip property and blocking resistance and oriented film of the composition.
Oriented polypropylene film is used in a wide variety of fields including food packaging, fiber packaging, etc. by virtue of its excellent transparency and mechanical properties. However, oriented polypropylene film, which has such excellent properties, is not free from shortcomings. In particular, when the sheets of the film are placed one upon another, they are apt to undergo the phenomenon of so-called blocking, that is, they tend to stick to one another; this greatly impairs the operability in packaging.
In order to improve the slip property and blocking resistance of polypropylene resin oriented film, there have been proposed methods which comprise kneading fine powders of inorganic substances as an antiblocking agent (hereinafter abbreviated as ABA) into polypropylene resin, forming the kneaded product into a sheet and then orienting the sheet. Such methods include the addition of zeolite, magnesium silicate, or the like to improve the blocking resistance (for example, JP-B-52-16134 and JP-B-48-14423) and the addition of silica fine powder to improve the blocking resistance (for example, JP-B-63-58170 and JP-A-4-288353). However, fine powders of inorganic substances are apt to undergo agglomeration; furthermore, since the affinity between polypropylene resins and inorganic substances is poor, voids are apt to develop in the oriented film during film orienting with the inorganic substances acting as nucleus, resulting in deterioration of film transparency.
In order to ameliorate the above-mentioned defects of the film incorporated with ABA of fine powders of inorganic substances, methods have been proposed which comprise dispersing ABA of high molecular substances in the form of fine powder into polypropylene resin, to obtain oriented film (for example, JP-A-57-64522, JP-A-5-214120 and JP-A-6-107868). However, these methods also have problems: When ABA having a small average particle diameter is used, it must be added in an amount of 1% by weight or more to obtain good slip property and blocking resistance and consequently a large number of fine surface irregularities are formed on the surface of the oriented film obtained, resulting in poor see-through impression of the film. Since the affinity between polypropylene resin and ABA of high molecular substance in poor, voids are apt to develop in the film during orienting, causing the deterioration of optical properties, so that ABA of high molecular substance cannot be used in a high concentration. Furthermore, since the degree of crosslinking of the ABA is insufficient, the blocking resistance of the resulting oriented film is unsatisfactory.
In view of such situations, the present inventors have made extensive study to improve the slip property and blocking resistance of polypropylene oriented film without greatly impairing its transparency. As the result, the inventors have found that a polypropylene resin composition for oriented film and the oriented film of the composition which meet the above-mentioned object can be obtained by using as ABA crosslinked polymer beads having a specific structure and thus attained the present invention.